Voice Over LTE (VoLTE) services have enjoyed increasing adoption by mobile network operators. As a result, there is a need for effective technical architectures for VoLTE roaming situations such as, but not limited to, when a subscriber of an operator in a first country is making use of mobile network services provided by a different operator in a different country. Initially, the 3GPP (The 3rd Generation Partnership Project) and the GSMA (GSM Association) specified a VoLTE roaming architecture called Local Break Out (LBO) (discussed in, for example, 3GPP TS 24.229), which is also referred to as RAVEL (Roaming Architecture for Voice over IMS with Local Breakout). Later, the GSMA endorsed a second VoLTE roaming architecture called S8 Home Routed (S8HR) (discussed in, for example, TR 23.749 for 3GPP Release 14).
Under the S8HR architecture, a visited LTE (Long Term Evolution) operator provides an LTE S8 interface to a subscriber's home public mobile network (HPMN, sometimes also referred to as an HPLMN, or home public land mobile network). All roaming calls (for voice or other services) are sent from the visited public mobile network (VPMN, sometimes also referred to as an VPLMN, or visited public land mobile network) to the HPMN as part of the S8 data payload. All IMS (IP Multimedia Subsystem) nodes are located at the HPMN, and all signaling and media traffic for the VoLTE roaming service go through the HPMN. Since IMS transactions are performed directly between a mobile station and a P-CSCF (Proxy-Call Session Control Function) in the HPMN, the VPMN becomes service-unaware at the IMS level. As a result, implementations of roaming policies by the VPMN can only be set up based upon IMSI (International Subscriber Mobile Identity, which includes MCC (mobile country code) and MNC (mobile network code) portions), APN (Access Point Name), and/or QCI (Quality of Service Class ID). However, such information is too limited to fully implement certain roaming agreements. Deep Packet Inspection (DPI) of IMS transactions by the VPMN cannot be relied upon to provide service awareness to the VPMN, as IMS transactions between a mobile station and a HPMN P-CSCF may be encrypted.